Nonvolatile semiconductor memory device and method of operating the same

1. A nonvolatile semiconductor memory device, comprising: a memory cell array having a plurality of electrically rewritable memory transistors arranged therein; and a control unit configured to govern control that, in order to erase data of a selected memory transistor, repeats a voltage application operation and a step-down operation, the voltage application operation applying an erase voltage to a body of the selected memory transistor to change a threshold voltage at which the selected memory transistor is conductive, and the step-down operation, in the case where a threshold voltage of the selected memory transistor has not changed to a desired value, decreasing the erase voltage by an amount of a certain step-down value, the control unit being configured to control the step-down operation to monotonically decrease the step-down value so as to reduce the value of the erase voltage close to zero as the number of times of the voltage application operations increases.

2. The nonvolatile semiconductor memory device according to claim 1 , wherein the memory cell array comprises a memory string configured having a plurality of electrically rewritable memory transistors connected in series therein, and the memory string comprises: a first semiconductor layer including a columnar portion extending in a perpendicular direction to a substrate, the first semiconductor layer functioning as a body of the memory transistor; a tunnel insulating film formed surrounding a side surface of the columnar portion; a charge storage film formed surrounding the tunnel insulating film and configured capable of storing a charge; a block insulating film formed surrounding the charge storage film; and a first conductive layer formed surrounding the block insulating film and functioning as a gate of the memory transistor.

3. The nonvolatile semiconductor memory device according to claim 1 , wherein a value by which the step-down value decreases is a constant value.

4. A method of operating a nonvolatile semiconductor memory device, the nonvolatile semiconductor memory device comprising a memory cell array having a plurality of electrically rewritable memory transistors arranged therein and a control unit configured to govern control of an operation to the memory transistor, in order to erase data of a selected memory transistor, the method comprising: repeating a voltage application operation and a step-down operation, the voltage application operation applying an erase voltage to a selected memory transistor to change a threshold voltage at which the selected memory transistor is conductive, and the step-down operation, in the case where a threshold voltage of the selected memory transistor has not changed to a desired value, decrease the erase voltage by an amount of a certain step-down value; and controlling the step-down operation to monotonically decrease the step-down value so as to reduce the value of the erase voltage close to zero as the number of times of the voltage application operations increases.